Non-mechanical steering of an optical beam is useful for many applications such as increasing the FOV in holographic projection. A PG can be used to non-mechanically steering the optical beam or an optical image. Theoretical and implementation aspects of the PG can be found in, e.g., “Polarization Gratings for Non-Mechanical Beam Steering Applications” authored by J. Buck et al. and published in Proceedings of SPIE 8395, Acquisition, Tracking, Pointing, and Laser Systems Technologies XXVI, 83950F (15 May 2012), the disclosure of which is incorporated by reference herein. The PG deflects a CP light beam by a deflection angle to a traveling direction of the CP light beam. The deflection is to the left side or the right side of the traveling direction. Whether the left or right side is selected in light deflection is determined according to the sense or rotation, or handedness, of the CP light beam.
However, there is a need for improvement in the PG regarding its steering resolution and response time. The steering resolution is generally poor as the PG offers a fixed deflection angle. The PG's response time of switching the light beam from the left side to the right side or vice versa, usually around 10 ms per steer, is relatively shorter than many other beam-steering devices such as a liquid prism. Despite this, a shorter response time, e.g., a sub-millisecond response time, is required for a number of practical applications such as tracking a fast-moving object and then spotlighting it in a radar application.
US2012/0188467 addresses the issue of poor resolution by disclosing a beam-steering device realized with a serial cascade of multiple units each being a PS followed by a PG. Each unit, with the PS thereof realized by nematic LC, is electronically controllable to deflect an incoming light beam or maintain its traveling direction by using an electric field to control orientation of LC molecules in the PS. By controlling an actual deflection angle in each unit, a plurality of achievable deflection angles is obtained so that the steering resolution is increased. Although the steering resolution is improved, the response time in beam steering is not shortened.
U.S. Pat. No. 9,575,366 discloses a FLC switch that is capable of fast switching and low-cost manufacturing, and claims that this switch is useful for implementing a diffraction grating. The disclosed switch includes a pair of crossed polarizers, causing no deflection of the incident beam when the disclosed FLC switch is switched on. Although the FLC switch of U.S. Pat. No. 9,575,366 provides fast switching and the multistage arrangement used in the beam-steering device of US2012/0188467 improves the beam steering resolution, applying the FLC switch of U.S. Pat. No. 9,575,366 to implement the PS in each unit of the beam-steering device of US2012/0188467 does not result in a workable PG having an improved steering resolution and a fast response time since the FLC switch is basically an on-off switch.
There is a need in the art for a new design of a beam-steering device that provides an improved steering resolution and a fast response time.